(a) Field of the Invention
The present invention relates to a switching mode power supply (SMPS) and, more particularly, to a quasi-resonant SMPS and a method of operation.
(b) Description of the Related Art
In general, a SMPS has at least one switching device (e.g., a transistor) which can be switched or turned on an off to deliver or supply power to a load. A SMPS can rectify an input AC voltage to an input DC voltage (also called a DC-link voltage) and convert the input DC voltage into an output DC voltage having a different level. An SMPS can be used in electronic devices such as mobile phone chargers or laptop computer adapters.
Electromagnetic interference (EMI) can arise in a SMPS, in part, due to turning on and off a switching device at constant frequency. A device with high EMI generates noise in other devices using the same power source. Various regulations exist for EMI, such as, for example, limiting the average value of measured EMI during constant time in the constant frequency range, or limiting the average value of measured EMI in the constant frequency range.
A quasi-resonant SMPS is one in which the switching frequency for turning on and off the switching device is modulated within a range in order to reduce EMI. The EMI spectrum for a conventional quasi-resonant SMPS is scattered with the changing switching frequency, responsive to a ripple of the DC-link voltage. This minimizes a power supply's switching losses, while allowing for higher switching frequencies. As the AC input voltage increases in a conventional quasi-resonant SMPS, the DC-link voltage ripple of the SMPS decreases. The decrease in the DC-link voltage ripple may reduce the modulation range of the switching frequency, thus resulting in an increase in EMI.